1. Field of the Invention
The present invention relates to a piezoelectric transformer driving circuit for igniting a cold cathode tube, and more particularly relates to the circuit which can obtain sufficient restart output when using PWM light modulation.
2. Description of the Related Art
Conventionally, a wire wound transformer is used in devices which require high voltage, such as a cold cathode tube, a copying machine, and an electrostatic remover.
In recent years, however, a piezoelectric transformer has started to be used for reasons that it can be miniturized and is more stable, etc. This piezoelectric transformer is used as a piezoelectric inverter, since its characteristics are suitable for igniting a cold cathode tube and it can be made thin.
In a piezoelectric inverter for igniting a cold cathode tube, a much higher output voltage is required at initial ignition than during continuous ignition. This is due to the characteristics of the cold cathode tube. For example, a cold cathode tube which can be continuously ignited at a voltage of between 200 to 300 V requires a voltage of more than 1 kV at initial ignition. An even higher voltage is required when the cold cathode tube has not been used for a while, or when it has been left in a cold place, and sometimes the cold cathode tube cannot be ignited by a single application of high voltage. As shown in FIG. 3, when the cold cathode tube ignites after a single application of high voltage, it can be continuously ignited at a constant voltage thereafter.
On the other hand, in case the cold cathode tube cannot be ignited after a single application of high voltage, a high voltage must be continuously or repeatedly applied to the cold cathode tube.
However, the stability of a miniaturized apparatus becomes a big problem when continuously generating a high voltage of more than 1 kV.
Accordingly, as shown in FIG. 4, a restart circuit is provided to control the output voltage over time and repeatedly output a high voltage. The high voltage is continuously output until the tube ignites.
The output voltage is time-divided by using a pulse in order to adjust the brightness (modulate the light) of the cold cathode tube when it is being continuously ignited. FIG. 5 shows the voltage output when the cold cathode tube is continuously ignited while adjusting its brightness.
In a circuit which adjusts brightness using a pulse as described above, the output of the restart circuit shown in FIG. 4 is also time-divided. As a result, only the cut output voltage shown in FIG. 6 can be obtained. Since the width of the pulse used in pulse light modulation is not synchronized to the cycle of the restart circuit, the number of maximum voltages generated is small. Shortening the cycle of the restart circuit would increase power consumption.
In the restart circuit, the secondary side voltage of the piezoelectric transformer is monitored, and, when it reaches the limit voltage, a control circuit reduces the output voltage of the piezoelectric transformer. In this case, drive delay and the like of the restart circuit and the piezoelectric transformer cause a delay in the feedback loop, and the output voltage reaches its maximum just before the brightness adjusting pulse turns OFF. Unless the limit is determined, the delay causes an abnormal voltage exceeding the limit voltage to be generated at the next ON.
It is an object of this invention to provide a piezoelectric transformer driving circuit in which the output of the piezoelectric transformer is controlled by a brightness adjusting pulse, whereby restart output can be continuously output without being cut and a cold cathode tube can be reliably ignited.
In order to achieve the above objects, this invention provides a piezoelectric transformer driving circuit comprising a piezoelectric transformer which boosts an input voltage to obtain a voltage required in igniting a cold cathode tube; a restart circuit which detects the voltage output from the piezoelectric transformer and allows the piezoelectric transformer to repeatedly output a high voltage required in initially igniting the cold cathode tube; a PWM controller which allows the piezoelectric transformer to intermittently output a voltage in order to adjust the brightness of the cold cathode tube; and a stopping unit which stops the control operation of the PWM controller when the restart circuit is operative.
According to this constitution of the invention, the PWM controller becomes operative when the restart circuit is operative. Therefore, the voltage output from the piezoelectric transformer is disjointed as shown in FIG. 6, but achieves the original continuously repeated output as shown in FIG. 4. The PWM controller becomes operative after initial ignition, obtaining an intermittent output voltage such as that shown in FIG. 5.